2-Keto-L-gulonic acid is an important intermediate for the production of L-ascorbic acid. This compound can be converted to ascorbic acid according to the well-known Reichstein method.
The fermentative production of 2-keto-L-gulonic acid from D-sorbitol or L-sorbose is known. For example. Japanese Patent Publication No. 40154/1976 discloses the production of 2-keto-L-gulonic acid from D-sorbitol by means of microorganisms of the genus Acetobacter. Bacterium or Pseudomonas, which are capable of oxidizing D-sorbitol under aerobic conditions, thus producing 2-keto-L-gulonic acid. The yields obtained by this process are however, rather low. namely less than 6 g/1.
According to another known process, which is disclosed in "Acta Microbiologica Sinica" 21(2), 185-191, (1981), 2-keto-L-gulonic acid can be produced from L-sorbose by means of a mixed culture of microorganisms composed of Pseudomonas striata and Gluconobacter oxydans, the yield being 30 g/1 when starting from a concentration of 70 g/1 of L-sorbose, and 37 g/1 when starting from a concentration of 100 g/1 of L-sorbose.
European Patent Publication No. 0221 707 discloses the production of 2-keto-L-gulonic acid from L-sorbose by means of Pseudogluconobacter saccharoketogenes with and without concomitant bacteria. However, the yield of this known process by means of Pseudogluconobacter saccharoketogenes per se is at most 55.3-87.6 g/1 (conversion ratio: 34.2-54.1%) (See: page 13, Table 4 of European Patent Publication No. 0221 707).
Furthermore, European Patent Publication No. 0278447 discloses a process for the production of 2-keto-L-gulonic acid from L-sorbose by means of a mixed culture of microorganisms, one of which having the identifying characteristics of DSM No. 4025 and the other one having the identifying characteristics of DSM No. 4026 (a Bacillus megaterium strain). While the microorganism having the identifying characteristics of DSM No. 4025 particularly OSM No 4025 itself has, as such, substantially no ability to grow and produce 2-keto-L-gulonic acid, the mixed culture, containing as the second component a Bacillus megaterium microorganism, does grow and does produce 2-keto-L-gulonic acid in yields of 40 g/1 and more. Thus, the presence of the second microorganism component (Bacillus megaterium) was considered to be essential for the operability of the process disclosed in the European Patent Publication No. 0278447.